1. Field of the Invention
The present invention relates generally to an apparatus for receiving broadcasting signals, and more particularly, to a broadcasting signal receiving apparatus which is operative to receive a digital audio broadcasting signal and to obtain a reproduced audio signal based on the digital audio broadcasting signal received thereby.
2. Description of the Prior Art
Although an analog audio broadcasting system which includes an amplitude-modulated (AM) audio broadcasting system in which audio signals are transmitted in the form of an AM audio information signal and a frequency-modulated (FM) audio broadcasting system in which audio signals are transmitted in the form of a FM audio information signal, has been put to practical use for a long time in the field of audio broadcasting, it has recently been proposed to introduce a digital audio broadcasting system in which audio signals are transmitted in the form of digital audio information signals for the purpose of improving the quality of audio information transmitted or received in the system. Especially, in Europe, the digital audio broadcasting system called "DAB" has been already put to practical use in some countries.
The digital audio information signal transmitted from a broadcasting station using the digital audio broadcasting system is called a digital audio broadcasting signal. The digital audio broadcasting signal carries not only audio information data forming a digital audio signal but also service information data representing service information, such as weather forecast, traffic information and so on, and further carries control information data which are necessitated for reproducing the digital audio signal based on the audio information data and the service information based on the service information data on the receiving side. Both the audio information data and service information data usually contain various kinds of program information data.
The audio information data and service information data contained in the digital audio broadcasting signal are usually subjected respectively to time interleaving arrangements for the purpose of minimizing deterioration resulting from data bit omission, data bit transformation and so on occurring on transmission. Each of the audio information data and service information data are transmitted in the form of a series of unit segments each having a time duration of, for example, 24 ms and constituting a logical frame. The time interleaving arrangement to which each of the audio information data and service information data are subjected is carried out on each interleaving completion segment consisting of successive sixteen logical frames. Accordingly, time interleaved audio information data or time interleaved service information data corresponding to each original group of successive sixteen logical frames constitute a new group of successive sixteen logical frames.
The logical frame consists of a series of, for example, 864 unit data each constituting a capacity unit (CU) which corresponds to 64 bits to form a minimum data segment. Accordingly, the logical frame corresponds to 64.times.864=55,296 bits. In case of the audio information data or service information data containing various kinds of program information data, the quantity of data contained in each of the logical frames constituting a single kind of program information data amounts to, for example, 140 capacity units (140.times.64=8,960 bits) at the maximum.
Such a digital audio broadcasting signal as described above is received by use of a digital audio broadcasting receiver. In the digital audio broadcasting receiver, each of the digital audio broadcasting signals transmitted respectively from a plurality of broadcasting stations is received selectively through a tuning operation by a tuner and the received digital audio broadcasting signal is subjected to a demodulation processing so as to produce control information data, time interleaved audio information data and time interleaved service information data. Each of the time interleaved audio information data and time interleaved service information data are subjected to a time de-interleaving arrangement to be released from the time interleaving arrangement to return the original audio information data or original service information data.
The time de-interleaving arrangement to which each of the time interleaved audio information data and time interleaved service information data are subjected is carried out usually in accordance with the following first and second steps. In the first step, one of various kinds of program information data contained in the time interleaved audio information data or time interleaved service information data are selected. In the second step, the selected program information data are subjected to a time de-interleaving arrangement to be released from the time interleaving arrangement to return the original program information data. As a result, the original program information data are obtained as time de-interleaved audio information data or time de-interleaved service information data.
Then, with the participation of the control information data, audio program data based on the time de-interleaved audio information data and service program data based on the time de-interleaved service information data are separately obtained. The audio program data are subjected to a decoding processing to produce a digital audio signal and a reproduced audio signal is obtained based on the digital audio signal. The service program data are also subjected to a decoding processing by which reproduced service data are obtained.
The time de-interleaving arrangement to which each of the time interleaved audio information data and time interleaved service information data obtained through the demodulation processing on the received digital audio broadcasting signal are subjected is carried out to each interleaving completion segment consisting of, for example, successive sixteen logical frames which are contained in each of the time interleaved audio information data and time interleaved service information data. Therefore, for example, a circuit structure as shown in FIG. 1 is used for such time de-interleaving arrangement as described above. The circuit structure shown in FIG. 1 comprises a program selector 11, a time de-interleaving portion 12, a random access memory (RAM) 13 forming a memory device and a control unit 14.
In the circuit structure shown in FIG. 1, composite data Dxd containing the time interleaved audio information data and time interleaved service information data which are obtained through the demodulation processing to which the received digital audio broadcasting signal is subjected, are supplied to the program selector 11. A program selection control signal Csp is also supplied to the program selector 11 from the control unit 14. In the program selector 11, a data selection by which one of various kinds of the program information data contained in the audio information data constituting the composite data Dxd or one of various kinds of the program information data contained in the service information data constituting the composite data Dxd are selected is carried out in response to the program selection control signal Csp and program information data Dpd selected by the data selection are derived from the program selector 11 to be supplied to the time de-interleaving portion 12.
In the time de-interleaving portion 12 and the RAM 13 connected to the time de-interleaving portion 12, first, a plurality of capacity units Dpu, each of which forms unit data of 64 bits and which constitute the program information data Dpd contained in successive sixteen logical frames derived from each of interleaving completion segments of the composite data Dxd, are successively stored in the RAM 13. Then, after the whole capacitor units Dpu forming the unit data constituting the program information data Dpd contained in those successive sixteen logical frames are once stored in the RAM 13, the capacitor units Dpu stored in the RAM 13 are read from the RAM 13 in a predetermined de-interleaving manner so as to produce time de-interleaved program information data Dpd' contained in new successive sixteen logical frames so that the time de-interleaving arrangement is carried out to the program information data Dpd.
The time de-interleaved program information data Dpd' are derived from the time de-interleaving portion 12 to be time de-interleaved audio information data or time de-interleaved service information data.
As described above; the time de-interleaving arrangement to which each of the time interleaved audio information data and time interleaved service information data are subjected is carried out to each interleaving completion segment consisting of, for example, successive sixteen logical frames. Therefore, when the time interleaved audio information data or time interleaved service information data are subjected to the time de-interleaving arrangement, a memory device used for the time de-interleaving arrangement, such as the RAM 13 shown in FIG. 1, is required. The Ram is necessary to provide an appropriate arrangement under any possible data condition, and to have such bit capacity as necessary for a time de-interleaving arrangement carried out on the time interleaved audio information data and time interleaved service information data, each of which has been subjected to a time interleaving arrangement carried out to each interleaving completion segment consisting of successive sixteen logical frames each provided with sufficient data to be in a full condition.
The bit capacity necessary for the time de-interleaving arrangement carried out on both the time interleaved audio information data and time interleaved service information data, each of which has been subjected to the time interleaving arrangement carried out to each interleaving completion segment consisting of successive sixteen logical frames in the full condition, is expressed with the formula : 64.times.864.times.16.times.n/2, provided that each logical frame contains successive 864 capacity units, each capacity unit consists of 64 bit, and n is the number of bits of soft decision in a Viterbi decoding processing for error correction to which time de-interleaved audio information data and time de-interleaved service information data are to be subjected, and n is usually selected to be 4 in the case of the audio information data and service information data.
Accordingly, the bit capacity necessary for the time de-interleaving arrangement carried out on both the time interleaved audio information data and time interleaved service information data, each of which has been subjected to the time interleaving arrangement carried out on each interleaving completion segment consisting of successive sixteen logical frames in the full condition is, for example, 64.times.864.times.16.times.4/2=1,769,472.apprxeq.1.77 megabits.
Generally, since RAMs supplied for wide use are used for constituting almost all of such memory devices as the RAM 13 shown in FIG. 1, a RAM having the bit capacity of 2 megabits or more than 2 megabits may be used for constituting the memory device having the bit capacity of 1.77 megabits to be used for the time de-interleaving arrangement. That is, it is usual that the memory device used for the time de-interleaving arrangement to which the time interleaved audio information data and time interleaved service information data are subjected, such as the RAM 13 shown in FIG. 1, has the bit capacity of 2 megabits or more.
However, in fact, in the time de-interleaving arrangement to which each of the time interleaved audio information data and time interleaved service information data are subjected, one of various kinds of program information data selected to be extracted from the time interleaved audio information data or time interleaved service information data are actually subjected to the time de-interleaving arrangement. The quantity of data contained in the selected single kind of program information data amounts to, for example, 140 capacity units, namely 64.times.140=8,960 bits, at the maximum, as aforementioned.
Accordingly, the bit capacity necessary for the time de-interleaving arrangement carried out on the selected single kind of program information data is expressed with the formula: 64.times.140.times.16.times.n/2, and consequently results in 64.times.140.times.16.times.4/2=286,720.apprxeq.0.3 megabits, provided that n equals 4.
This means that, although the memory device provided previously for the time de-interleaving arrangement to which the time interleaved audio information data and time interleaved service information data are subjected, such as the RAM 13 shown in FIG. 1, has the bit capacity of 2 megabits or more, only the bit capacity of 0.3 megabits or less of the bit capacity of 2 megabits or more, namely about one-sixth or less of the bit capacity of 2 megabits or more, is actually used for storing the unit data (capacity units) constituting the selected single kind of program information data.
Even if two of various kinds of program information data are selected to be extracted from the time interleaved audio information data or time interleaved service information data for the time de-interleaving arrangement, the bit capacity of the memory device used for storing the unit data constituting the selected two kinds of program information data comes to 0.3.times.2=0.6 megabits or less and this bit capacity is considerably small compared with the bit capacity of 2 megabits or more of the memory device.
As described above, as for the memory device provided previously for the time de-interleaving arrangement to which the time interleaved audio information data and time interleaved service information data are subjected, such as the RAM 13 shown in FIG. 1, a major part of the bit capacity thereof is not actually used and therefore efficiency of utilization is very low.